Display arrangement for electric oscillographs



June 6, 1950 R. F. CLEAVER DISPLAY ARRANGEMENT FOR ELECTRICOSCILLOGRAFHS 2 Sheets-Sheet 1 Filed May 14, 1947 gum 15 R. F. cLEAvER 93 DISPLAY ARRANGEMENT FOR ELECTRIC OSCILLOGRAPHS Filed May 14, 1947 2Sheets-Sheet 2 A llorney Patented June 6, i950 nrsrnar nalvesrmm' eonornrc oscnaoa Richard Francis Cleaver, London, England, assignortolnternational Standardlllectric Corporation, New York, N. Y acorporation or Dela Application May 14, 1947, Serial No. 747,943 InGreat Britain May 1%, 1946 system having fixed receiving antennas, aradial trace is produced on the screen of a cathode ray oscillograph,and the angular position or the trace gives the bearing of atransmitting or beacon station. The position of the trace is determinedby the ratio of two voltages, which are proportional to the strengths ofthe signals induced in two antenna combinations. The position of thetrace is aiiected by any inaccuracy in the con struction of theoscillograph; and according to present practice, the errors caused bythe inaccuracy are minimised by individual calibration of theoscillographs. This process is rather costly, and is furthermoreundesirable in view of the limited life of oscillographs. I

Another source of error inthis system is the It is the principal objectof the present inven-- tion to overcome these difiiculties by means ofan arrangement in which the necessary accuracy of reading is obtainedwith any oscillograph without the necessity for calibrating it, and inwhich parallax may be completely eliminated.

The invention accordingly Provides a display arrangement for an electricoscillograph comprisin means for periodically producing a firstindication on the screen of the oscillograph in accordance with anunknown quantity of given type to be measured, an adjustable deviceadapted to produce a comparison quantity of. like type and ofany-desired known magnitude within a given range, means for periodicallyproducing a second indication on the said screen in accordance with thesaid comparison quantity, the said second indication alternating withthe first indication, and means for adjusting the said device in suchmanner as to bring the two said indications into registration on thesaid screen whereby the value of the unknown quantity is given by thatof the known comparison quantity.

Such a display arrangement may clearly'be as employed in any system inwhich measurements are made from the indication or trace produced on anoscillograph screen, the position of such indication or trace beingdetermined by the quan-. tity to be measured. It is, however, ofparticular application to a radio direction finding system;

andthe embodiment which willfbe described to illustrate theinvention issuch a system.

The embodiment is illustrated on the accompanying drawings in which:

Fig. 1 shows a. schematic circuit diagram of the embodiment;

Fig. 2 shows details of an shown in Fig. 1;

Fig.- 3 shows a front view of the preferred method of arranging theoscillograph'and scales on which the bearings of aircraft or the likeare indicated and measured;

Fig. 4 shows a section of the lines 44 of Fig. 3; and Fig. 5 shows adiagram of a potentiometer which may be used in Fig. 1.

The embodiment to be described is a radio direction finding systemadapted to be installed on a vessel such as an aircraft carrier fordeterelectronic switch the bearings of aircraft or other vessels in thevicinity of the vessel. It can also be used mdetermining the bearing ofthe vessel itself with respect to a fixed beacon station. Thisembodiment'merely illustrates one application of the invention, whichcan be employed in quite other circumstances which are not related todirection finding systems. Fig. 1 shows a radio direction finding systemi of known character not shown in detail. It

will be supposed to include the necessary antenna system for receivingwaves from an aircraft or other transmitting station and for derivingtherefrom in two output circuits 2 and 3 in known manner deflectingvoltages whose ratiov is substantially proportional to the tangent ofthe bearing angle of the transmitting station. These defiecting voltagesare applied to an indicating cathode ray oscillograph d of which onlythe two pairs of deflecting plates 5, 6 and l, 8 are shown. Thisoscillograph is of conventional type, and the electron gun and operatingarrangements are not shown.

The output circuits 2 and 3 of the direction finding system I areconnected to the plates of the oscillograph 6 through a periodicallyoperated change-over switch 9 to which is also connected a calibrator "Iprovided accordin to the invention to produce a cursor spot or trace onthe oscillograph screen. The calibrator l0 includes a squarepotentiometer l I having a pair of straight contactors l2 and I3 adaptedto rotate about an axis I4 passing through the centre of the square.This potentiometer is formed from four similar, straight, uniformresistance elements l5. l6, II, It, arranged to form a square ABCD inthe manner described in thespeciflcation of copending application N0.720,529, filed January 7, 1947, now abandoned, and is adapted to producein two output circuits l9 and 20 corresponding voltages whose ratio isequal to the tangent of the angle 0 between the line of the contactorsl2 and I3 and 0A.

The switch 9 connects the plates 1,3 and 6, 6 respectively, of theoscillograph 4, alternately to the output circuit 2 and 3 of thedirection finding system I, and to the output circuits I9 and 20 of, thecalibrator Hi. This switch may operate at a frequency of 100alternations per second, for example, and has been showndiagrammatically. It could be a mechanical switch of any suitablepattern, but it may also be an electronic switch, preferably of the kinddescribed in the specification of co-pending application No. 724,318,filed January 25, 1947, and illustrated in the accompanying Fig. 2,which is substantially a reproduction of Fig. 1 of that specification.The action of this switch will be explained later.

The diagonal corners A and B of the potentiom eter ll (Fig. 1) areconnected to a series circuit consisting of two fixed resistances 2| and22, and an adjustable resistance 23., while the comers C and D areconnected to a similar series circuit of two fixed resistances 24 and25, and a resistance 26 which may also be variable. Resistances 2|, 22,24 and 25 should preferably be equal, and resistances 23 and 26 shouldbe similar. a

The two contactors l2 and i3 are fixed to an insulating member 21 sothat each passes through the axis I 4 about which they rotate. Thecontactors are connected respectively through corresponding parts of aswitch 28 to terminals 29 and 33, to which an appropriate direct currentsource (not shown) is intended to be connected. A potentiometer 3| withits movable contact connected to ground may be connected between theterminal 29 and 30 for the purpose of balancing the direct currentsource to ground.

The circuit of the calibrator III will be seen to be the same as thecircuit of Fig. 3 of application No. 720,529 mentioned above, exceptthat the alternating current source is replaced by a direct currentsource. It is shown in that specification that if the resistances 23 and26 of Fig. 1 accompanying this specification are equal, if resistances2|, 23, 24 and 25 are also equal, and if E1 and En are the voltagesobtained at the outputcircuits l9 and 20 respectively, then E2/Ei=tan 0.The potentiometer H is provided with a scale of angles, and acorresponding index by means of which the angle 0 may be accuratelyread. A preferred form of such arrangements will be described withreference to Figs. 3 and 4.

When the switch 9 is thrown to the right-hand side, a single spot willbe obtained on the oscilloof the oscillograph. When the switchalternates in the manner described, the two spots will be seen together.If resistances 23 and 26 be coupled so as to be simultaneously adjustedin such manner that equal maximum values of E1 and E: are produced (thatis, when the angle 0 of the potentiometer is 0 and respectively), thenunder this condition it will be possible to adjust these resistances andthe potentiometer ll so that the two spotscoincide. Under theseconditions both spots are equally afiected by the deflectioncharacteristics of the oscillograph which therefore cancel out, and then=0, and the bearing of the transmitting station will be obtained withhigh accuracy from the reading of the potentiometer. It is clear,therefore, that this result is not affected by the errors of theoscillograph. Furthermore, all parallax has been eliminated.

It is clear, however, that if the deflecting voltages derived fromcircuits 2 and 3 of the direction finding system I are not accuratelyrelated according to the tangent law as a result of errors in thesystem, or for other reasons, a bearing error will be produced. Incertain circumstances to be described later, the resistances 23 and/ or23 may be separately adjusted, or the potentiometer l I may be modified,so that the voltages E1 and F4 are related ina manner which departssimilarly from the tangent law, by which means the error may becompensated.

Single spots on the oscillograph screen are, however, not veryconvenient to work with, and it is far preferable to derive radial linetraces from the deflecting voltages in the manner explained in thespecification of co-pending application No. 667,259, filed May 4, 1946,now Patent No. 2,502,428. Inthe preferred form of the switch 9 shown inFig. 2. such arrangements have been included. In what follows, the spotor trace derived from the calibrator ID will be called an electroniccapsor."

- The resistance elements l5, l6, l1 and it which make up thepotentiometer ll. may be similar strips of insulating material closelyand evenly wound with resistance wire, or they may be moulded fromresistance material and they may be arranged in any of the mannersdescribed in application Number 720,529.

Fig. 2 shows the preferred form of the switch 9, but only the circuitconnected to one of the. plates 5 of the oscillograph 4 is shown. Thecircuits for the other plates are similar as will be explained.Conductors 3A and 20A of Fig. 1 will be respectively connected toterminals 32 and 33 of Fig. 2. The terminals 32 and 33 are respectivelyconnected to the plate 5 of the oscillograph through resistances 34 and35, and through blocking devices consisting of diodes 36 and 31. Thediodes have their cathodes connected to the plate 5, and are arranged tobe alternately unblocked by pulses from a multivibrator of conventionalpattern consisting of two similar valves 33 and 39 arranged so that theanode of each is cross-connected to the control grid of the otherthrough respective condensers 40 and 4 I.

The cathodes are directly connected to the ground terminal 42 (which isalso the negative terminal of the high tension source for the valves),and the anodes are connected to the .positive high tension terminal 43through resistances 44 and 45. The control grids of the valves 36 and 39are grounded through resistances 46 and 41. This is a well knownarrangement and may be made to generate rectangular pulses whoserepetition frequency and duration depend aarao'ra oscillograph 3 isconnected to ground through a resistance 50.

Since the unblocking periods of the diodes 36 and 31 should preferablybe equal, the elements associated with the two valves 38 and 39 shouldpreferably be respectively equal, so that the condenser chargingcircuits will have equal time constants.

In order to explain the action of the circuit of Fig. 2, it will beassumed that the multivibrator has been so designed that the potentialof the anodes of the valves 38 and 39 with respect to ground alternatesbetween the two values +100 volts and +600 volts at a frequency of 100cycles per second, for example, and that the potentials applied to theterminals 32 and 33 do not vary outside the limits +100 volts and +500volts. When the potential of the anode of valve 38 is +600 volts, thediode 58 will be blocked and the positive potential at terminal 33 willbe applied through the diode 36 to the plate 5, since this diode will beunblocked in this condition. If the resistance 50 is large compared withthe resistance 34, there will be a negligible potential drop in theresistance 36. Under these conditions the diode 49 will have a potentialof +100 volts on its cathode from the anode of the valve 39, and thiswill hold the potential of the anode of the diode 49 substantially at+100 volts, so that the diode 31 will be blocked, thus effectivelydisconnecting terminal 32 from the plate 5. It will be evident that whenthe multivibrator switches over, the diodes 35 and 39 will be blockedand the diodes 31 and 38 will be unblocked, thus applying the potentialat terminal 32 to the plate 5 instead of that at terminal 33. It will beclear, therefore, that the potentials at terminals 32 and 33 will bealternately applied to the plate 5 for equal periods of time at afrequency of, say, 100 cycles per second.

It will be understood that each of the defiecting plates of theoscillograph I will be provided with elements similar to the elements32, 33, 35, 35, 36, 31, 48, 49 and 50, arranged in a similar manner, allofwhich are operated from the one single multivibrator which ispreferably not duplicated. These extra elements have not been shown inorder to avoid complicating the figure. In such a case all the diodescorresponding to 35 will be connected to the anode of the valve 38 andall the diodes corresponding to 39 will be connected to the anode of thevalve 39. The three other terminals such as 32 correspondingrespectively to the plates 5, I and 8 are connected to conductors 33, 2Aand 2B, respectively, of Fig. 1; and the three other terminals such as33 corresponding respectively to these plates are connected toconductors 203, NA and NB. Then the spot corresponding to the potentialsapplied to all the terminals similar to 32 from the direction findingsystem i will be produced alternately with the spot corresponding to thepotentials applied to all the terminals similar to 33 from thecalibrator ID (the switch 29 be closed), and if the switching rate ishigh enough (e. g. cycles per second) the two spots will is appear toexist simultaneously on the oscillograph screen.

As already mentioned, however, it is very desirable to join each spot tothe centre of the screen by a straight line trace. This may be done byproviding condensers 5i and 52 respectively connecting the anodes of thediodes 36 and 31 to ground, and similar condensers (not shown) for eachof the other plates of the oscillograph. The relatively slow charging ofthe condensers 5i and 52 through the resistances 34 and 35 will causethe spots to describe radial straight lines, in the manner explained inthe specification of co-pending application No. 667,259, filed May 4,1946, the condensers 5i and 52 being discharged when the correspondingdiodes 4B and t9 conduct.

In a slightly modified form of Fig. 2, the diodes may all be reversedand the resistance 50 may be connected to the positive high tensionterminal 53 instead of to ground. In this case it is the negativechanges of the anode potentials of the valves 38 and 39 which performthe switching, instead of the positive changes. The arrangement has anadvantage when the indication of the oscillograph 5 has to betransmitted over a line for repetition on a distant oscillograph by theuse of cathode follower amplifying circuits in the manner described inthe specification of copending application No. 667,259. The advantage isassociated with the fact that the sudden potential changes caused by theshort-circuiting of the condensers 5i and 52 by the diodes 48 and 69 arepositive changes, which are preferable for the most efilcient operationof the cathode follower amplifiers.

It will be clear that, if desired, any other suitable type of one-wayblocking devices capable of being unblocked by applied potentials may beused instead of the diodes 36, 31, 68, and 49, such for example as dryplate rectifiers, or triodes or like devices with grid electrodes whichmay be controlled by the pulses from the multivibrator.

Figs. 3 and 4 show one preferred form of the mechanical arrangements fordetermining the bearing from the traces on the oscillograph screen. Itis assumed that the condensers 5| and 52 have been included in order toobtain the preferred radialtraces, as explained.

The potentiometer ll (Fig. l) is enclosed in a horizontal box 53, and isprovided with a vertical spindle 53 mechanically connected to theinsulating member 21 which carries the contactors l3 and I3. To thespindle 54 is keyed a horizontal bevel gear wheel 55 which meshes withan equal vertical bevel gear 56 formed on the back of a rotatable ringmember 51 which co-axially surrounds the screen 58 of the cathode raytube 5. The front of the ring member 51 carries a bezel which holds afiat circular glass plate 59 just clear of the screen 58. To operate thedevice, the plate 59, or the ring member 51, is rotated by hand, causinga corresponding rotation of contactors I2 and I3. Two narrow annularscale plates 69 and 5| are co-axially arranged to surround the screen58, one inside the other, just behind the glass plate 59. Neither ofthese plates is mechanically connected with the ring member 51,.and theinner plate 60 is fixed. Plate 60 carries two scales, 0-l80 on thestarboard side and 0-l80 on the port side, the corresponding points 0and on the two scales coinciding and being marked F (for Fore) and A(for Aft), while the graduations port 90 and starboard 90 are designatedP and S respectively. The outer plate rotates under the control of theThe glass plate 59 carries a radial index line 63 on the back surface.This line should be of suflicient length to overlap both scale plates,but

preferably should not extend into the central area of the glass platethrough which the screen 66 is visible. It will be evident that theindex line 63 eflectively indicates the angle of the potentiometer II onthe scale plate 66 subject to an appropriate zero adjustment which willbe described later.

The glass plate 59 is also provided with a small central circle 64engraved on the back surface by means of which the oscillograph tracesmay be approximately centered with respect to the scale plates 66 andGil Assuming that the apparatus is operating normally (the switch 28(Fig. 1) being closed), and that an aircraft within range istransmittting bearing indicating waves, two radial straight line .traces65 and 66 will be seen on the screen. The

trace 66 (which does not necessarily register with the index line 63) isthe electronic cursor and is produced by the calibrator ill in themanner explained. The trace 65 corresponds to the aircraft. In orderaccurately to measure the bearing of the aircraft, the ring 51 (or plate59) is rotated by hand, rotating the contactors i2 and I3 (Fig. 1) atthe same time, so that the electronic cursor 66 rotates about the centreof the screen until and/or from the site have been compensated byadjusting the direction finding equipment to bring the bearing trace 65into registration with the electronic cursor 66, approximate bearingscan be read of! rapidly using trace 66 and scale plate 60 only, theelectronic cursor being ignored or switched off by means of switch 28.The adjustment of the resistances 23 and 26 will not have to be repeateduntil the oscillograph 4 is replaced.

The adjustment for quadrantal-instrumental error (and quadrantal siteerror, if any) should be made as follows, with the usual means providedin the direction finding system, making use of a test oscillator orother transmitter having a known bearing (port say) with respect to theships head. The plate 59 is adjusted as before so that the index line 63reads port 45 on the scale plate 60. Then the quadrantal adjustmentshould be made until the trace 65 exactly coincides with the cursortrace 66. When this has been done, the bearing of any aircraft may beaccurately determined by adjusting the cursor trace to coincide with thetrace corresponding to the aircraft, the bearing with respect to theship's head being obtained by reading the index line 63 on the scale 60,or with respect to true north, on the scale 6|.

It will be seen that all errors due to the oscillograph cancel out, andthere is no parallax to eflect the accuracy of the readings. Whencoincidence of the two-traces is obtained, the ratio v of the twovoltages at the outlets 20 and IQ of the it coincides exactly with thetrace 65, or with its mean position if it fluctuates. The bearing with Irespect to the ships head is then obtained accurately by reading theposition of the index line 63 on the scale 60, or with respect to truenorth. on the scale 6 I.

Certain preliminary adjustments, which will now be described, arenecessary. It will be assumed that the mechanical coupling between the 1insulating member 21 (Fig. 1) through the spindle 54 and bevel gears hasbeen set so thatthe index line reads zero on the scale plate 60 when theangle 0 of the potentiometer II is zero. This may be done as accuratelyas desired.

- In order to obtain approximate readings without the help of theelectronic cursor, it will be convenient to centre the oscillograph 4with re-' spect to the ring member 51, and to set the oscillograph sothat the effective deflection axes of the two pairs of plates correspondsubstantially with the F-A and P-S graduations on scale plate 66. Theseadjustments do not affect the accuracy when measuring bearings with theaid of the electronic cursor. It is also convenient, though notessential, to compensate approximately for the quadrantal error of theoscillograph 4 (due to any inequality between X- and Y- platesensitivities) by setting the index line 63 to read port 45 (say) onscale plate 66 and adjusting resistances 23 and 26 so that theelectronic cursor trace 66 also reads port 45 on scale plate 60, asnearly as may be judged. The accuracy of this adjustment is notimportant and the reason for making it is to ensure that when quadrantalerrors arising in the direction finding equipment calibrator I6 (Fig. 1)must be equal to the ratio of those at ,the outlets 3 and 2 of thedirection finding system I, so that the angle 0 of the potentiometermust be equal to the bearing angle of the transmitter, provided thedirection finding system I itself is free from error.

As stated above, it is convenient to be able to make rapid approximatebearing observations without using the electronic cursor; the reason forthe initialcentering and alignment of oscillograph 4 and the adjustmentof resistances 23 and 26 will now be clear-they ensure that the angularposition of the cursor trace 66 registers approximately with index line63 at all settings of the calibrator. Provided that instrumental and seterrors have been compensated the angular position of the trace 65 willbe an approximate measure of the bearing, which can therefore be readapproximately from scale plate 60.

In cases where the direction finding system is subject to quadrantalsite error and is not provided with the means for correcting it, theprocedure described above can be modified so that a correction isapplied automatically when the electronic cursor is used. The error willnot in this casebe corrected when approximate bearings are measureddirectly by reading the angular position of trace 65 against scale plate60.

The method can best be understood by recalling that with the preferredsystem already described,

oscillographic quadrantal error was 'first eliminatedapproximately sofar as it affected the cursor trace 66, by adjustment of resistances 23andv 26. The means provided for quadrantal compensation in the directionfinding equipment I were then adjusted to cause the trace 65 to coincide1 with trace 66 when the relative bearing ofa transmitter was port45(say), with the calibrator 53 set to make the angle 0 (Fig. 1) equal toport 45. It now the direction finding equipment is not provided withmeans for quadrantal compensation it is possible'to achieve the sameresult by introducing a quadrantal error into the electronic cursorindication equal to the sum of the quadrantal errors of the directionfinding equipment and of the site. It will be appreciated that readingstaken without the aid of the electronic cursor will then be subject toquadrantal error equal to the algebraic sum of the quadrantal components4 introduced by the site, the direction finding equipment and theoscillograph.

The'alternative procedure is as follow After centering and aligning thedeflection axes of oscillograph 4 as before, the calibrator is set toport 45 (say) and resistances 23 and 26 are adjusted to bring the cursortrace 66 into coincidence with bearing trace 65 when a signal on abearing of port 45 is being received. The new adjustments of resistances23 and 26 will naturally differ from those obtained in the preferredmethod or adjustment. To show how this difference results in aquadrantal compensation, consider the simple case in which resistances 2l, 22, 2s and 25 of Fig. 1 are all equal, and resistances 23 and 2B arealso equal. Under these conditions tan =E2/E1. Suppose that theresistance 26 is adjusted to be K times the resistance 23, where Kdiffers from 1 by a small fraction. Let the angle 0 be efiectivelyincreased to 0+d0 thereby. Then it can be shown by simple trigonometrythat d0= (K-1) sin 20 approximately. Thus do is an error of quadrantalform and can be made to compensate any quadrantal error in the site orD/F system by suitable choice of K- that is, by adjustment of theresistance 26.

Another but less convenient method is to deform the square ABCD (Fig. 1)into a rhombus of which the angle CAD is 211. Then it can be shown thatd0= (cot a-l) sin 20, approximately. In this case, of course, theresistances 23 and 26 will be adjusted to be equal.

A similar expedient may be adopted to correct the octantal error whichmay arise in the antenna system of the direction finding system. Theresistance elements of the square are bent into the form of there-entrant equilateral octagon shown in Fig. 5. It can be shown that inthis case d0: b sin 40/ (1+sin 20+cos 20) approximately, where b is theacute angle between a side of the octagon and a side of the originalsquare. The denominator only varies about as 0 varies from 0 to 45, sothat de is octantal and nearly sinusoidal in form. Since the octantalerror arising in an antenna system depends on the frequency, thecompensation can only be partial in .any case, so thatthis approximationis likely to be good enough. The values of da obtainable are quiteconvenient; thus, for example, if b=12 the maximum value of (Z6 is about2.5". It will be understood that the calibrator ill of Fig. 1 could bearranged in various other ways. While the type of potentiometer I Iwhich has been described is preferred, some other known device may beused to produce deflecting voltages (or currents) which characterise anangle or other comparison quantity whose value is known and can beindicated independently of the oscillograph on a scale which isassociated with the device. The oscillograph could be provided withdeflecting coils for electromagnetic deflection, instead of the platesshown. The potentiometer ii could take any of the forms described inapplication No. 720,529 mentioned above, which may be suitable for thePurpose.

While it is probably best to arrange so that the traces 65 and 66 shownin Fig. 3 can be brought into coincidence, they could alternasince theinaccuracies due to curvature of the screen (and other causes) do notairect the measurements.

2. Cheaper oscillographs can be used for the above reasons, and nocalibration of the oscillograph is necessary.

3. No recalibrations 6r readjustments are necessary when an oscillographis changed (except when it is desired to take rough measurements withoutthe cursor as explained above).

4. Certain types of errors to which a direction finding system is liablecan be automatically compensated by the calibrator.

What is claimed is:

1. A display arrangement for a cathode ray oscillograph comprising meansfor producing a first pair of voltages whose ratio is determined by anunknown angular magnitude, a potentiometer adapted to produce a secondpair of voltages whose ratio is determined by the angular position ofthe contacting member of the said potentiometer, periodically operatingmeans for alternately applying the said first and second pairs ofvoltages to the deflecting plates of the oscillograph in such manner asalternately to produce corresponding radial straight line traces on theoscillograph screen, and means for adjusting the said contacting memberin such manner as to bring the two traces into coincidence whereby thecorresponding angular position gives the value of the unknown angularmagnitude.

2. An arrangement according to claim 1 in which the potentiometercomprises four straight, equal, uniform, resistance elements arranged ina square, and a pair of straight contacting mem= bers arranged in astraight line passing through and rotatable about an axis through thecentre of the said square.

3. An arrangement according to claim 2 comprising a direct currentsource connected to the said contacting members, and means for derivingthe said second pair of voltages respectively from the correspondingpairs of diagonal corners of the bridge.

4. An arrangement according to claim 3 comprising means forindependently adjusting the magnitudes of the said second pair ofvoltages.

5. An arrangement according to. claim 4 in which the said periodicallyoperating means comprises a periodically operating electronicchange-over switch.

6..An arrangement according to claim 5 in which the switch comprisescircuits including rectiflers through which the said pairs of voltagesare respectively applied, and means for periodically blocking andunblocking the said rectiflers in such manner as alternately to applythe said first and second pairs of voltages to the said deflectingplates.

7. A display arrangement for a cathode ray oscillograph comprising meansfor producing a first pair of voltages whose ratio is determined by anunknown angular magnitude, a potentiometer adapted to produce a secondpair of voltages whose ratio is determined by the angular 11 position orthe contacting member or the said potentiometer, periodically operatingmeans for alternately applying the said first and second pairs ofvoltages to the deflecting plates of the oscillograph in such manner asalternately to produce corresponding radial straight line traces on theoscillograph screen, and means for adfor the purpose of indicating thesaid angular position, on the said scale plate.

8. An arrangement according, to claim 7 in which the said scale plate isfixed and in which the said index comprises a radial line carried on aglass plate rotatable in front of the oscillograph screen and co-axiallywith respect to the scale plate, the said glass plate being mechanicallycoupled to the said contacting member or members.

9. A radio direction finding system comprising a display arrangementaccording to claim 8, means for receiving waves transmitted from astation, and means for deriving from the said waves the said first pairof voltages whose magnitude is determined by the direction to the saidstation.

10. A radio direction finding system for a vessel comprising a displayarrangement according to claim 9, a second movable annular scale platearranged co-axially with respect to the fixed scale plate and graduatedin degrees of angle and adapted to co-operate with the said index,

the said potentiometer is modified in such manner as to introduce anerror oi! like periodic iorm into the said second pair of voltages ofsuch magnitude as substantially to correct the said systematic errorwhen .the said traces have been brought into coincidence.

12. A system accordingto claim 11 subject to a quadrantal site error inwhich the said modified potentiometer comprises four straight, equal,uniform resistances arranged to form a rhombus having its angles sochosen as substantially to compensate the said quadrantal error.

RICHARD FRANCIS CLEAVER.

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